We have just shown in CrCl\(_3\)  that the ratio of its net inter-planar to net intra-planar exchanges per Cr\(^{3+}\)  spin is \(3\times 10^{-3}\) . This ratio is a few orders of magnitude smaller than \(\sim 9\times 10^{-1}\)  in CrI\(_3\)  [1]}, [2]}. Since spin wave energies of CrCl\(_3\)  will hardly change upon inclusions of the inter-planar exchanges, the excitations observed in this work must be virtually identical to the magnons in the monolayer limit. In the case of CrI\(_3\) , 2D magnon energies have been observed by means of Raman scattering in the monolayer limit [3]}. These energies are largely consistent with those of the bulk crystals when the interplanar exchanges are excluded. To observe magnon energies in CrI\(_3\)  close to the 2D limit, we performed inelastic neutron scattering measurements on the two polycrystalline samples with different amounts of thinning. As previously described, the sample S is mechanically reduced down close to the 2D limit. The momentum-integrated neutron scattering intensities of the sample L and S, respectively, at \(T\)  = 2 K are plotted in Fig. REF . Commonly in both samples, the powder-averaged excitation spectra exhibit two distinct bands corresponding to the upper and lower magnon modes, respectively, expected for honeycomb ferromagnets. Their overall band structures are equivalent to that of CrCl\(_3\)  (see Fig. REF ) although the excitations in the sample S are significantly broader in energy. It thus suggests that CrI\(_3\)  still retains its in-plane honeycomb structure after the ball milling.
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